In the design and manufacture of electronic devices that are operated within the microwave frequency range, i.e., frequencies greater than 1 GHz, the electrical interconnection between circuits is of great concern. An interconnection must be treated as a controlled-impedance transmission line when the interconnection dimension becomes a significant fraction of the signal wavelength (typically 1/10 to 1/8 of the wavelength is considered significant). At microwave frequencies nearly all circuit interconnections meet this criterion and, consequently, must be designed as controlled-impedance transmission lines. Interconnections which do not match the characteristic impedance of the circuit result in signal reflections and, thus, loss of power transmission and signal integrity.
Microwave-environment transitions between circuits within an electronic device may be made by coaxial connectors which provide electrical communication between two microwave assemblies housed within separate metallic boxes. Such structures are typically used for microwave hybrid packaging up to 26 GHz. However, a separate coaxial transition is needed for each electrical signal that must be conducted between the assemblies. Additionally, the machined metallic boxes and coaxial connectors add significantly to the manufacturing cost of the electronic device.
While they are not in common use, microwave assemblies which are surface mountable to a printed circuit board are known. This technique employs a surface mountable microwave integrated circuit assembly which uses transmission lines on a printed circuit board in place of coaxial plumbing between the milled assemblies. A backside coplanar waveguide is connected via reflow soldering. One difficulty with this solder-attach technique involves the differences in the thermal coefficients of expansion of the components. These thermal coefficient differences, which occur during the solder reflow process and, subsequently, during circuit operation, induce stresses at the solder joints that are directly proportional to circuit size. Hence, larger circuits have larger stresses. When the stresses surpass the fracture stress for the solder joint, failure of the electrical contact occurs. Thus, soldered, surface-mountable, microwave circuit assemblies must be limited in size or must be used only on interconnection substrates with matched thermal expansion. Moreover, the solder-attach technique does not facilitate replacement testing of integrated circuit assemblies.
For high-speed digital circuits, the required bandwidth for interconnections can be related to the rise time of the digital pulse by the following formula: EQU BW=0.35N/t.sub.r
where t.sub.r is the digital pulse rise time and N is the highest order frequency harmonic to be passed. Values of N=3 to 5 are typically used to estimate an adequate interconnect bandwidth for digital pulse integrity. With pulse rise times now below 500 picoseconds in many high-speed digital designs, it is imperative to design digital circuit interconnects with microwave bandwidths.
Another feature of high-speed digital circuits is the higher integrated circuit densities which typically require a greater number of interconnections than do conventional microwave circuits. Presently, there are now high-speed digital circuits which require up to 500 high-speed signal interconnections for optimum performance. One approach to satisfy this requirement for a high density of controlled-impedance interconnects is an aggressive development of multichip modules. By placing a plurality of chips on a substrate which can provide a high density of controlled-impedance lines, multichip modules eliminate the uncontrolled-impedance interconnections associated with single chip assemblies. An additional requirement for high-speed digital circuits is the interconnection of a large number of high-speed signals to multichip modules. These signal interconnects must also have controlled impedances.
In addition to the need for controlled-impedance interconnects, microwave and high-speed digital circuit assemblies have other requirements. These requirements include: (1) isolation of high frequency circuitry from electrical interference; (2) electrical reliability; and (3) mechanical reliability. For circuits which operate with significant power, the dissipation of thermal energy through the use of heat sinks may also be a requirement. For many high frequency circuits, it is also desirable to provide a hermetically-sealed environment. It would be advantageous if high frequency assemblies could provide these requirements while also providing ease of assembly, rework, and test. Preferably, these requirements can be met at a reduced cost. Further advantages would be realized if the packaging scheme allowed for flexibility in designing custom circuits while maintaining a standardized interconnect geometry between the circuit and the printed circuit board. These advantages include shorter design cycles and lower costs through the leveraged use of standard package elements and fixtures.
It is an object of the present invention to provide a controlled-impedance interconnection between a transmission line of a high frequency circuit and a transmission line of a substrate with the above requirements and enhancements. The high frequency circuit and the substrate can each be fabricated with either thick-film, thin-film, printed circuit board or multichip module technology. The high frequency circuit and the interconnection substrate can be components of either a microwave system or a high-speed digital system.